P 32. A comparison of TMS induced electric fields over multiple cortical areas using the finite element method

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Introduction Transcranial magnetic stimulation (TMS) has proven to be a powerful non-invasive technique in the field of neuroscience and in clinical studies. The first applications were developed for stimulation of the motor cortex (M1), but nowadays many cortical areas have been studied with TMS. Although this widespread usage over all possible cortical areas, most of the protocols are still based on the standards developed for M1. In the specific case of M1 stimulation a motor evoked potential (MEP) can be measured with the use of electromyography (EMG). The minimal intensity needed to evoke such an MEP is called the motor threshold (MT) and it varies between individuals. This threshold is used to adapt the stimulation intensity in single pulse, paired pulse or repetitive stimulation protocols per individual. Because most of the cortical areas outside M1 do not have a similar outcome measure like the MEP, the MT found over M1 is commonly used. There are, however, next to intra-individual differences also inter-individual differences in anatomy and physiology between cortical areas. These differences are for example the distance to the cortex, the thickness of the several tissue layers or the local cortical folding. Therefore, the intensity found over M1 can be sub-optimal for the cortical areas outside M1. By including the inter-individual differences between M1 and other cortical areas for the determination of the stimulation intensity, the induced TMS effects could possibly be optimized. However, before TMS protocols can be adapted, a verification of these inter-individual differences is advisable. A way to perform this verification is by TMS simulations. Objectives Compare the TMS induced electric field for multiple cortical areas with the induced electric field found for M1 stimulation. Materials and methods To simulate the TMS induced electric field a bioelectric problem has to be solved for a volume that represents a human head. We constructed a highly realistic head model based on Magnetic Resonance Imaging (MRI) and Diffusion Tensor Imaging (DTI) data. This model includes 8 tissue types and brain anisotropy. To solve the bioelectric problem the finite element method (FEM) was used. The electric field was calculated for multiple cortical locations, including cerebellum and frontal areas. The locations were based on experimental studies. The strength of these induced fields were then compared with the strength of the field over M1. Results The results show that the magnitude of the induced electric field differs largely between cortical locations, as expected ( Fig. 1 ). The distance between the cortical location and the coil has the most prominent effect on the electric field magnitude, but also the local anatomy and conductivity has an influence that cannot be ignored. Especially, the cerebellar locations ( Fig. 1 B) and the locations along the sagittal midline ( Fig. 1 C) display a field strength that is influenced by the surrounding tissue distribution. Conclusion The results indicate that an increase in intensity for cortical areas more distant from the coil is needed to induce a similar electric field magnitude as for M1. However, a correction in stimulation intensity solely based on the distance between the TMS coil and the cortex will probably not suffice. However, at this moment in time the optimal stimulation intensity per individual for locations outside M1 can only be determined with TMS simulations.